The Gorlice–Tarnów offensive during World War I was initially conceived as a minor German offensive to relieve Russian pressure on the Austro-Hungarians to their south on the Eastern Front, but resulted in the Central Powers' chief offensive effort of 1915, causing the total collapse of the Russian lines and their retreat far into Russia. The continued series of actions lasted the majority of the campaigning season for 1915, starting in early May and only ending due to bad weather in October.
The primary cause of the Central Powers victory was their superior artillery firepower. German heavy field guns and howitzers with their high angle of fire suppressed the Russian artillery and devastated their infantry counterattacks. The Germans improved the accuracy of their artillery fire through aerial observation. German siege mortars reduced Russian fortresses within days. German light artillery supported the infantry through direct fire, devastating Russian strongpoints.
In the early months of war on the Eastern Front, the German Eighth Army conducted a series of almost miraculous actions against the two Russian armies facing them. After surrounding and then destroying the Russian Second Army at the Battle of Tannenberg in late August, Paul von Hindenburg and Erich Ludendorff wheeled their troops to face the Russian First Army at the First Battle of the Masurian Lakes, almost destroying them before they reached the protection of their own fortresses as they retreated across the border.When these actions petered out in late September, much of two Russian armies had been destroyed, and all Russian forces had been ejected from the Masurian Lakes area of modern north-east Poland after losing almost 200,000 killed or captured soldiers.
The Russians did far better in the south where they faced the Austro-Hungarians, who mobilized more rapidly and started their own offensive in late August from Galicia, their province in partitioned Poland, initially pushing the Russians back into what is now central Poland. However, a well-executed Russian counter-stroke in late September, when they had brought more men to the front, pushed their enemy back over their own borders in disarray, leaving a large garrison besieged in the fortress city of Przemyśl.
The Germans came to their aid by forming a new Ninth Army which advanced from German Silesia into Poland in the Battle of the Vistula River. Although initially successful, the attack eventually petered out and the Germans returned to their starting points, as they retreated destroying the Polish railways and bridges to make it harder to invade German Silesia.
The Russians repaired the damage and then were poised to invade. The German Ninth Army was redeployed to the north, allowing them to put serious pressure on the Russian right flank in what developed as the Battle of Łódź in early November. The Germans failed to encircle the Russian units, and the battle ended with an orderly Russian withdrawal to the east near Warsaw, the German occupation of Łódź, and the end of the immediate threat to Silesia.
In fierce winter fighting General Franz Conrad von Hötzendorf, the chief of staff of the Austro-Hungarian Army, attacked the Russians who had forced their way into the Carpathian passes in the south of Galicia. Both sides suffered appallingly, but the Russians held their line. By this time half of the Austro-Hungarian Army that had entered the war were casualties. Conrad pleaded for additional German reinforcements to hold the passes. German Chief of Staff Erich von Falkenhayn refused, but in April 1915 Conrad threatened a separate peace if the Germans would not help. Conrad and Falkenhayn met and planned a joint strike on the Russian left flank at the far southern end of the Eastern Front, in the Gorlice-Tarnów front,130 km (81 mi) southeast of Kraków. A successful advance from there would force the Russians to retreat from the passes to save themselves from being cut off.
German intelligence detected no signs of an imminent Allied attack on the Western Front. Moreover, their field army was still growing. They were removing an infantry regiment from each division, leaving them with only three, but not reducing the numbers of essential divisional specialists, a better allocation of forces for an artillery war. Each reconfigured division was reinforced with 2,400 new men, recruited since the outbreak of the war, who were dispersed among the veterans. The released infantry regiments were formed into 14 new reserve divisions.
Conrad had to bow to Falkenhayn’s conditions. The joint attack would be by an Austro-German Army Group commanded by a German, whose orders from Falkenhayn would be transmitted via the Austro-Hungarian command. The Group would contain the Austro-Hungarian Fourth Army (eight infantry and one cavalry divisions) under Archduke Joseph Ferdinand, an experienced soldier. The Germans formed a new Eleventh Army made up of eight divisions, trained in assault tactics in the west. They were brought east on 500 trains. The Army was led by the former commander of the German Ninth Army, General August von Mackensen, with Colonel Hans von Seeckt as chief of staff. Mackensen, whose political sensitivities had been polished as an adjutant to the Kaiser, would also lead the army group. They would be opposed by the Russian Third Army (18½ infantry and five and a half cavalry divisions, under General D. R. Radko-Dmitriev).
Mackensen was provided with a strong train of heavy artillery commanded by Generalmajor Alfred Ziethen, which included the huge German and Austro-Hungarian mortars that had crushed French and Belgian fortresses. Airplanes were provided to direct artillery fire, which was especially important since ammunition was short on both sides: only 30,000 shells could be stockpiled for the attack. Another significant plus was the German field telephone service, which advanced with the attackers, thereby enabling front-line observers to direct artillery fire. To increase their mobility on the poor roads, each German division was provided with 200 light Austro-Hungarian wagons with drivers.Falkenhayn moved German Supreme Headquarters, OHL (Oberste Heeresleitung), to Pless in Silesia, an hour's drive from Austrian headquarters. To prevent spying, the local inhabitants were moved out of the buildup area. In the north the German Ninth and Tenth armies made diversionary attacks that threatened Riga. On 22 April, the Germans launched the first poison gas attack near Ypres, divulging what might have been a decisive weapon merely to distract the Allies in the west. Mackensen had ten infantry and one cavalry divisions (126,000 men, 457 light guns, 159 heavy pieces, and 96 mortars) along the 42 km (26 mi) length of the breakthrough sector. Facing him were five Russian divisions consisting of 60,000 men but desperately short on artillery. For firesupport the Russians could only count on 141 light artillery pieces and four heavy guns.
And one of the four burst as soon as the battle began.The Russian supreme commander, the Grand Duke Nicholas Nicholaevitch, learned that Germans had arrived on their flank but did not make a counter-move.On 1 May, the Central Powers’ artillery opened harassing fire, zeroing in their guns. The following morning at 0600 they began a sustained bombardment, at 0900 the heavy howitzers joined in. The huge mortar shells were especially terrifying, their blast killed men tens of meters from the explosion. The Russian fortifications were "... more ditches than trenches." so they were easily smashed and their feeble barbed wire belts torn apart by howitzers firing high explosive. At 1000 the Austro-German infantry attacked in thick skirmishing lines. Mackensen’s orders were for his entire front to move forward as one, regardless of local opposition: each unit was set a minimum distance to advance each day. If a machine gun held them up, a field gun was brought up to destroy it. When driven back the Russians almost invariably counterattacked in dense formations, only adding to their losses.
Central Powers (arrayed north to south):
Austro-Hungarian 4th Army (Austro-Hungarian units unless otherwise indicated):
Combined Division “Stöger-Steiner”;
XIV Corps (German 47th Reserve Division, Group Morgenstern, 8th & 3rd Infantry Divisions);
IX corps (106th Landsturm & 10th Infantry Divisions);
In reserve behind IX Corps: 31st Infantry Brigade (“Szende Brigade”), 11th Honved Cavalry Division.German 11th Army (German units unless otherwise indicated):
Guards Corps (1st & 2nd Guards Divisions);
Austro-Hungarian VI Corps (39th Honved Infantry & 12th Infantry Divisions);
XXXXI Reserve Corps (81st & 82nd Reserve Divisions);
Combined Corps “Kneussl” (119th and 11th Bavarian Infantry Divisions);
In reserve: X Corps (19th & 20th Infantry Divisions).Austro-Hungarian 3rd Army,
X Corps (21st Landsturm, 45th Landsturm, 2nd Infantry & 24th Infantry Divisions)Russian 3rd Army (north to south):
IX Corps (3 militia brigades, 3 regiments of 5th Infantry Division, 2 militia brigades, 3 regiments of 42nd Infantry Division, 70th Reserve Division, 7th Cavalry Division [in reserve]);
X Corps (31st Infantry & 61st Reserve Divisions, 3 regiments of 9th Infantry Division);
XXIV Corps (3 regiments of 49th Infantry Division, 48th Infantry Division & 176th (Perevolochensk) Infantry Regiment of 44th Infantry Division);
XII Corps (12th Siberian Rifle Division, 12th & 19th Infantry Divisions & 17th (Chernigov) Hussar Regiment);
XXI Corps (3 regiments of 33rd Infantry Division & 173rd (Kamenets) Regiment of 44th Infantry Division);
XXIX Corps (Brigade of 81st Infantry Division, 3rd Rifle Brigade, 175th (Batursk) Infantry Regiment of 44th Infantry Division & 132nd (Bender) Infantry Regiment of 33rd Infantry Division);
11th Cavalry Division.Behind the Russian front lines:
Scattered across the rear of 3rd Army:
3rd Caucasus Cossack Division, 19th (Kostroma) Infantry Regiment of 5th Infantry Division, 33rd (Elets) Infantry Regiment of 9th Infantry Division; 167th (Ostroisk) Infantry Regiment of 42nd Infantry Division;Army Reserve:
Brigade of 81st Infantry Division, 3 regiments of 63rd Reserve Division, Composite Cavalry Corps (16th Cavalry Division (less 17th Hussar Regiment), 2nd Consolidated Cossack Division); 3rd Don Cossack Division
Radko Dimitriev quickly sent two divisions to stem the Austro-German breakthrough, but they were utterly annihilated before they could even report back to headquarters. From the Russian point of view, both divisions simply disappeared from the map. On 3 May the Grand Duke Nicholas was sufficiently concerned to provide three additional divisions and to authorize a limited withdrawal . The attackers surmounted the first major geographical obstacle, the Wisloka river, on a captured bridge. By 5 May the attackers were through the three trench lines that had opposed them, by 9 May they had reached all assigned objectives. Grand Duke Nicholas permitted a limited withdrawal, but rejected advice to construct a well fortified position far behind the frontline and then to pull back to it. At this point the Russian counterattacks grew ever more desperate, often throwing brand new recruits into battle, some armed only with grenades or wooden clubs. The Austro-Hungarian Third and Fourth Armies pressed forward in the Carpathian passes, the Russians retreated before them while they still might. On 12 May a conference at Pless decided that Mackensen should continue to advance to the San River and take bridgeheads on the east bank. Sustaining the attack required meticulous organization: relieving surviving but worn-out infantry, moving forward artillery, ammunition, and all other supplies along roads and rail lines that had to be repaired as they advanced. Each new assault followed the pattern of the first, a hail of artillery fire blasted a passageway for the infantry.
When Army Group Mackensen reached the San his front was more than 150 km (93 mi) from his rail-heads, as far as they could go until the newly reconquered railways were operating again. Once this was done they established bridgeheads over the San on 16 May. On the east bank the old city of Przemyśl was surrounded by 44 forts. After a prolonged siege its Austro-Hungarian defenders had surrendered it –for a second time— on 22 March. On 30 May Eleventh German Army’s artillery began to duel with the guns in the forts. The huge mortars easily smashed the concrete. On 1 June the infantry occupied three large forts. A Russian counterattack failed. Two days later the victors marched into Przemyśl, the Austro-Hungarian troops were cheered exuberantly by its citizens, and the triumph triggered high-spirited celebrations throughout Austro-Hungary. The same day the Austrian Fourth and Seventh armies struck the flank of the Russian Eleventh Army, driving for the River Dniester.
Falkenhayn provided replacements to bring the depleted Eleventh Army ranks back close to their initial strength. The Russians also reinforced their defenders. Lemberg, the Galician capital, was set as the next objective, 100 km (62 mi) further east. An attack on 13 June sent the Russians into a headlong retreat and on 21 June the Grand Duke Nicholas ordered them to abandon Galicia. On 22 June Mackensen’s Austro-Hungarians entered Lemberg after an advance of 310 km (190 mi), an average rate of 5.8 km (3.6 mi) per day. The Galician oil fields, crucial for the German navy, were soon back in production and 480,000 tons of badly-needed oil was captured.The Russian Third Army left about 140,000 prisoners in enemy hands, and almost ceased to exist as a fighting unit. The 3rd Caucasian Corps, for example, brought up to establishment of 40,000 men in April, was reduced to 8,000. It was thrown into the battle on the San against the Austrian First Army, and succeeded in taking some 6,000 prisoners and nine guns, but one of their divisions was down to 900 men by 19 May.
Seeckt proposed that now the Eleventh Army should advance north towards Brest-Litovsk, with their flanks shielded by the rivers Vistula and Bug. Hindenburg and Ludendorff agreed and proposed that simultaneously their Tenth and their new Nieman army should take Kovno and then drive toward Vilna. With the Germans in both Vilna and Brest all the major railway lines from Poland to Russia would be cut. The Russian Army in the Polish salient would be snared in a pocket; such a massive defeat might bring peace. Falkenhayn decided that this bold plan exceeded their means and instead ordered frontal attacks all along their present front in Poland.
The Grand Duke Nicholas issued orders that yielded to the pressure step by step, evacuating both Galicia and the Polish salient to straighten out their front line, hoping to buy the time to acquire the weapons they so desperately needed, for example 300,000 rifles. This enormous movement is known as the Great Retreat of 1915. Warsaw was evacuated and fell to the new Twelfth German Army on 5 August, and by the end of the month Poland was entirely in Austro-German hands.The victors asked the Danes to offer to host a peace conference. Tsar Nicholas refused to participate: he had pledged his allies not to make a separate peace. Mackensen continued to lead Austro-German armies throughout the war, first conquering Serbia and then occupying Romania. The Tsar himself replaced the Grand Duke Nicholas as supreme commander.